CNC Machining vs Injection Molding
CNC machining and injection molding solve the same problem — turning a CAD model into real parts — at opposite ends of the volume curve. The real question is whether you need a handful of accurate parts fast, or thousands of identical plastic parts at the lowest possible per-piece cost.
The verdict
Choose CNC machining for low-to-mid volumes (one-off to a few hundred), fast iteration, tight tolerances, metal parts, and designs that are still changing. Choose injection molding once your design is frozen and volumes are high enough to spread the tooling cost across thousands of plastic parts — then per-part cost drops to cents. CNC wins on speed-to-first-part and flexibility; molding wins on unit economics at scale.
Side-by-side data
| Attribute | CNC Machining | Injection Molding |
|---|---|---|
| Best volume range | 1 – ~1,000 parts | ~1,000 – millions |
| Tooling / setup cost | Low (fixturing & programming only) | High (mold: ~$2k–$80k+) |
| Per-part cost (low vol) | Lower | Very high (tool not amortized) |
| Per-part cost (high vol) | Higher (each part re-cut) | Very low (cents per part) |
| Lead time to first parts | ~1 – 10 days | ~3 – 8 weeks (tool build) |
| Typical tolerance | ~±0.025 – 0.05 mm | ~±0.05 – 0.2 mm (shrink-dependent) |
| Materials | Metals & rigid plastics | Thermoplastics (+ some thermosets) |
| Surface finish | Tool marks; finishing optional | Smooth as-molded, texturable |
| Design changes / iteration | Edit CAD & re-run | Costly — may need new/modified tool |
Which should you choose?
Choose CNC Machining if…
- You need parts in days, not weeks — prototypes, jigs, fixtures, or urgent spares.
- Volumes are low to mid (one-off through a few hundred) and tooling can't be amortized.
- The part is metal, or a rigid engineering plastic that mills well.
- Tolerances are tight and you can't tolerate mold shrink or warp.
- The design is still evolving — you want to edit CAD and re-cut without scrapping a tool. Not sure? Try the Process Selector or estimate cost with the CNC Cost Estimator.
Choose Injection Molding if…
- You're producing thousands to millions of identical plastic parts.
- The design is frozen and validated — you're ready to commit to a tool.
- You want the lowest possible per-part cost and fast cycle times.
- You need molded-in features: living hinges, snap-fits, textures, overmolding, color.
- Consistent cosmetic surface finish across a high-volume run matters. Browse candidate resins in the materials library.
Key differences that matter
- Cost structure: CNC has near-zero tooling but a fixed cost per part that doesn't fall much with volume; molding front-loads a large tool cost, then per-part cost collapses — so the two cost curves cross somewhere in the hundreds-to-thousands range.
- Lead time: CNC can deliver first parts in days because there's no tool to build; molding needs weeks for the mold before a single part ships, though steady-state output is then extremely fast.
- Geometry rules: CNC is subtractive and limited by tool access (deep pockets, internal undercuts are hard); molding needs draft angles, uniform wall thickness, and gate/ejector planning to avoid sink and warp.
- Tolerance & finish: CNC holds tighter tolerances directly off the cutter; molded parts shrink as they cool, so the tool is sized to compensate — repeatable once dialed in, but looser part-to-part out of the gate.
- Flexibility: a CAD change is cheap for CNC and expensive for molding, where modifying hardened steel may mean welding, re-cutting, or a new tool — the reason teams prototype on CNC first.
Need CNC or injection-molded parts made?
Use our free tools to pin down your process and spec, then get a quote from a vetted factory.
Open the Process SelectorGet a Quote →Frequently asked questions
At what volume does injection molding beat CNC machining?
There is no single number, but the crossover for plastic parts is often somewhere in the hundreds to low thousands of pieces. Below that, CNC usually wins because it has no tooling cost; above it, molding's very low per-part cost overtakes the upfront tool investment. The exact break-even depends on part size, geometry, material and how the tool is built (aluminum vs. hardened steel).
Is CNC machining more accurate than injection molding?
Generally yes. CNC routinely holds tolerances around ±0.025–0.05 mm and can go tighter, with no shrink or warp to manage. Molding accuracy is typically a bit looser (often ~±0.05–0.2 mm) because the polymer shrinks as it cools, though a well-built tool and stable process can be very repeatable across millions of shots.
Can I use CNC for prototypes and then switch to molding for production?
Yes, and it is a very common path. Teams CNC-machine the first parts to validate fit, function and design changes with no tooling lock-in, then commit to an injection mold once the design is frozen and volumes justify it. Bridge tooling (aluminum molds) can sit in between for early production runs.
Which is better for metal parts?
CNC machining is the default for metal parts across nearly all volumes. Injection molding is for thermoplastics (and some thermosets); metal equivalents like die casting or metal injection molding (MIM) exist but are separate processes with their own tooling and design rules.
Values, ranges and break-even points are typical/indicative for early-stage guidance only and vary widely by shop, part geometry, material, tooling strategy and specification. Confirm critical specs and quantities with an mfgiq engineer before committing to production.